1. Field of the Invention
This invention relates generally to a method and apparatus for ablating tissue, and more particularly, it relates to a method and apparatus for ablating tissue which results in a smooth reshaping of, e.g., a cornea of an eye, with an ablating laser beam by directing the laser beam to incremental ablation points or spots in a predetermined pattern on the tissue to be ablated.
2. Description of the Prior Art
The ablation of human tissue with an ablating laser beam is known. Typically, a laser beam is incrementally scanned across a layer of tissue to be ablated, and adjacent points or spots on the layer of tissue are sequentially ablated to reshape the tissue.
For example, laser ablation is utilized to reshape a cornea of an eye. The human eye functions much like a camera, with a lens in front and a light-sensitive screen, the retina, in the rear. Images enter the eye through the cornea which is a transparent domed window located at the front of the eye. In a normal-visioned eye, the cornea bends or refracts incoming images, causing the images to focus on the retina. The inability of the cornea to refract incoming images properly causes blurred vision and is called a refractive disorder. Myopia (near sightedness) is one of the most common refractive disorders. In a nearsighted eye, the cornea is too steep, causing an image to be focused in front of the retina, and distant objects to appear blurred.
To correct near sightedness, laser vision correction (Photorefractive Keratectomy or PRK) can be performed to make the cornea less steep by removing a microscopic layer of the cornea through a gentle reshaping technique. The laser produces an invisible beam of ultraviolet light which removes microscopic amounts of corneal tissue at adjacent points in a scan without causing damage to surrounding cells.
To achieve most corrections, a microscopic amount of corneal tissue approximately a third the thickness of a human hair--is sculpted from the surface of the cornea. The laser beam is directed onto the surface of the eye in a series of adjacent pulses along a linear path. Each pulse removes approximately one quarter of a micron of tissue (one hundred thousandths of an inch). The reshaping is successful by removing the layers of the cornea which cause distant objects to be focused more properly on the retina of the eye. The precision of a laser beam assures total control of the correction process with the integrity and the strength of the cornea being preserved.
Conventional laser ablation for vision correction has achieved good results, with the majority of patients no longer dependent on corrective lenses after the treatment. However, it is important to achieve a smooth ablation such that that remaining tissue does not contain any significant ridges or other rough areas. Accordingly, there exists a need for a method and apparatus for performing laser vision surgery which accurately and effectively ablates tissue such that a smooth surface remains. Improved apparatus and methods for achieving a smooth ablation, such as improved ablation patterns, are needed.